ABSTRACT BACKGROUND Obstructive sleep apnea (OSA) is associated with an increased risk of cardiovascular morbidity. A driver of this is metabolic dysfunction, and in particular diabetes mellitus (T2DM). Prior studies identifying a link between OSA and T2DM have excluded subjects with undiagnosed T2DM, and there is a lack of population-level data on the interaction between OSA and glycemic control among diabetics. We assessed the relationship between OSA severity and T2DM prevalence and control in a large multinational population. METHODS We performed a cross sectional analysis of 6,616 participants in the European Sleep Apnea Cohort (ESADA) study, using multivariate regression analysis to assess T2DM prevalence according to OSA severity, as measured by the oxyhemoglobin desaturation index. Diabetics were identified by previous history and medication prescription, and by screening for undiagnosed diabetes with HbA1c measurement. The relationship of OSA severity with glycemic control was assessed in diabetic subjects. RESULTS T2DM prevalence increased with OSA severity, from 6.6% in subjects without OSA to 28.9% in those with severe OSA. Despite adjustment for obesity and other confounding factors, in comparison with subjects free of OSA, patients with mild, moderate or severe disease had OR(95% CI) of 1.33(1.04-1.72), 1.73(1.33-2.25) and 1.87(1.45-2.42)(p<0.001) respectively for prevalent T2DM. Diabetic subjects with more severe OSA had worse glycemic control, with adjusted mean HbA1c levels 0.72% higher in severe OSA patients than in those without sleep disordered breathing (ANCOVA p<0.001). CONCLUSIONS Increasing OSA severity is associated with increased likelihood of concomitant T2DM, and worse diabetic control in patients with T2DM.